Para-xylene and ortho-xylene are important petrochemical intermediates that are used to produce end products such as polyester fibers and film, plasticizers, polyester, and alkyd resins.
Currently, para-xylene is produced mainly by the isomerization of the isomeric C.sub.8 -aromatic hydrocarbons, namely ortho-xylene, meta-xylene, and ethylbenzene, or nonequilibrium mixtures of these isomers (including para-xylene) thereof, into the para-xylene isomer. The isomerization is typically effected by contacting the C.sub.8 hydrocarbons, in admixture with hydrogen, with a dual function catalyst possessing both hydrogenation and cracking activities, thereby effecting the desired isomerization reaction. Common operating conditions are temperatures from about 0.degree.-700.degree. C., pressures of about atmospheric to 100 atmospheres, and a hydrogen to hydrocarbon mole ratio of about 0.5-25.
An example of a C.sub.8 aromatic isomerization process is U.S. Pat. No. 3,078,318 (issued to Berger). The Berger patent discloses a process for the selective production of a particular isomer which comprises subjecting a C.sub.8 aromatic hydrocarbon fraction to isomerization in the presence of a catalyst comprising platinum on alumina, and thereafter separating out a particular xylene from the resulting hydrocarbon mixture. Further, the Berger patent teaches that after removal of a particular isomer from a mixture of C.sub.8 aromatic hydrocarbons, the remaining isomeric components in the mixture may be subjected to isomerization in the presence of hydrogen and a catalyst comprising a Group VIII metal on alumina to regenerate the mixture of C.sub.8 aromatic hydrocarbon isomers and approach the equilibrium proportions of ortho-, meta-, and para-xylenes and ethylbenzene.
In Meyers, Robert A., Handbook of Petroleum Processes, pages 5-68 to 5-70, McGraw-Hill, Inc. (1986), an integrated xylene isomerization process is described. In this process, a deheptanized C.sub.8 aromatic feedstock containing a mixture of C.sub.8 xylene isomers and ethylbenzene is passed to a xylene splitter fractionation unit to remove heavy aromatics and recover the desired amount of ortho-xylene product. The overhead from this column goes to the para-xylene recovery section. Effluent, depleted in para-xylene, exits from the xylene recovery unit and is directed to the isomerization unit where xylene isomers are isomerized to equilibrium and ethylbenzene is converted to benzene and ethane. The effluent from the isomerization reactor is separated into a hydrogen-rich vapor and a liquid phase which is passed to a deheptanizer to remove C.sub.7 minus products.
In the isomerization reactor, ethylbenzene reacts to produce benzene and ethane and the nonequilibrium xylene isomer mixture moves towards equilibrium. During the course of these reactions, there is some loss of xylenes by transalkylation and dealkylation to other aromatics, for example, toluene and C.sub.9 aromatics. Further, there are additional xylene losses due to cracking and saturation of C.sub.8 aromatics. It is important to reduce the xylene losses in the isomerization reactor because this reduces feedstock requirements for the xylene isomerization process and increases the proportion of higher-valued products that can be recovered.
In the past, the problem of xylene losses due to side reactions occurring in the isomerization reactor was addressed by manipulating catalyst formulations to increase selectivity (xylene retention) of the C.sub.8 aromatic isomerization process. Numerous catalysts for isomerizing C.sub.8 aromatics have been disclosed, and many of them involve the use of crystalline-aluminosilicate compounds known as zeolites. Zeolites particularly suited for isomerization include mordenite and the ZSM variety. In addition to the zeolite component, certain metal promoters and inorganic oxide matrices have been included in isomerization catalyst formulation. Examples of inorganic oxides include silica, alumina, and mixtures thereof. Metal promoters, such as Group VIII or Group III metals, have been used to provide a hydrogenation functionality.
Another method disclosed for reducing the xylene loss in a C.sub.8 aromatic isomerization process is isomerizing at less severe operating conditions. The problem with this approach is that a reduction in ethylbenzene conversion per pass usually results when these less severe operating conditions are employed.
There is a need for a method for reducing the xylene losses that accompanies C.sub.8 aromatic isomerization while maintaining high ethylbenzene conversion.